Mirrors are not to be believed. You stand in front of them, knowing that you can't trust yourself as an arbiter of truth, so you turn from side to side, thinking that maybe, if you snap your head around quickly enough, you can actually see yourself as others see you. It never works, of course. You stare and you turn, and you wonder: Am I fat?

"If you've ever been fat, you will either be fat for the rest of your life or you will worry about being fat the rest of your life." I came across those words 20 years ago in the play "Fighting International Fat," by Jonathan Reynolds. A pretty obscure place to find the underlying thesis of your waking life, but one doesn't get to choose. That casual observation struck me with the profound power of its obvious truth, much like Kafka's observation, "The meaning of life is that it ends." Kafka did not then add, "...but once you're dead, you won't gain weight." Which I would have found comforting.

I was, in fact, a fat kid, and terribly self-conscious about it, although some people who knew me as a child knit their brows in confusion when I tell them this. To them, I might have been a tad chubby; but from behind my eyes I was a freak, slow and rubbery and comical, picked last for any team, forced to shop in the husky section of the boy's clothing department to which my mother took me for semiannual torture sessions. At summer camp, I was known as Pumpkin, which became Plumpkin, which I accepted at the time as my due. I even laughed about it. What was I supposed to do? Run away?

Eventually, at the age of 15, that's what I did. I lost almost 40 pounds in that first manic bout of running, going from chubby to slender to skinny to my panicked mother bringing me to the doctor. He tested me and found me to be in excellent shape, if a bit bony—running seven miles a day will do that to you—and told me to be careful I didn't lose too much weight, and told my mother not to worry.

Of course, I didn't mention to him how I was so hungry all the time that I couldn't think of anything but food. As people who fast—voluntarily or not—will tell you, hunger takes over your brain, which is why I would skip lunch at school but end up reading cookbooks in the library. I would go to bed dreaming of my skimpy breakfast, half a grapefruit and coffee with Sweet and Lo, and spend the rest of the day mentally concocting recipes for things I would never, ever allow myself to eat. And for all that fasting and running and self-denial, I would still take off my shirt at the end of every day—or the middle of the day, if the bathroom had a mirror and a lockable door—and look at my protruding ribs and delineated hips and say to myself: Still too fat. When my heart breaks for that kid I once was, it's because he had transformed himself into what he had always yearned to be, a thin person. And he was too crazy to know it.

That period ended as adolescence gave way to college, and other pleasures—beer, friends, theater, writing, and most of all, girls—stole my focus and wrecked my asceticism. It's hard to deny yourself in one area when you're desperate to indulge in all the others. And so I gained most of the weight back, and stopped running for a while, and ever since then have engaged in the weight wars, getting happy, getting fat, becoming miserable, getting skinnier again, and always fruitlessly seeking the truth of what I was from the silent mirrors.

My midlife running boom began the way my first one did. A doctor weighed me for a physical, and as he blithely noted down "200 lbs.," he didn't notice my repressed scream. I started running again. I ran a race, and then another, and then marathons, and then Boston Marathons, and I've lost 30 pounds (okay, as of this morning 27 pounds), and I'll be damned if I ever gain them back again.

Or I should say: More damned. The obsession with weight—far, far more common than you might suppose among amateur athletes—is a curse. "I run to eat," we say, but we're not so much taking pleasure in the food our running earns us but in the immunity it grants us. We hit the box of doughnuts at work or the side of fries with lunch, telling ourselves, "I did my six this morning," and we feel, for a moment, that we are Normal People, able to indulge in sweets and fats without suffering the kind of inner guilt and recrimination usually reserved for embezzlers.

It's not about being fat. I know people of all shapes whose sense of self is blessedly untethered from their weight. It's about the terror of what we might become if we allow ourselves to let go, to get weak, to slow down. I run now for a lot of reasons, for fitness and for times and for friendship and for the sheer pleasure of motion. But deep inside I know I'm also running because with every step, I'm leaving Plumpkin further behind. And I'm afraid if I ever stopped, he'd catch me, and consume me in his unending appetite, and I'd have to look back into the mirror from behind his frightened eyes.

Peter Sagal is a 3:27 marathoner and the host of NPR's Wait, Wait...Don't Tell Me! For more, go to runnersworld.com/scholar.